I'm less sure about current production chips, but the older chips used lead carbonate, which is a white powder, in the resin. It's the same stuff they used to make paint in the old days. It wouldn't appear as flecks, but would change the overall density of the resin and make it appear more dense in an X-ray. I've used a lead test on some of my old chips and got a strong positive result.

I don't see any problem with someone wanting to fill a cancelation hole in a chip in their collection for cosmetic reasons, but if the repair was done with a lead filled resin to pass the X-ray test, then that would seem to be a deliberate attempt to deceive.